Adding dropup support to split buttons and navbar

* Documents the hidden .dropdown-menu.bottom-up option (to be used for dropups)
* Extends the .bottom-up class to carets and all relevant caret styles
* Adds .dropdown-menu.pull-right class for right-aligned dropdowns and dropups
* Adds examples and some information to the "split dropdown" and "navbar" sections of the Docs

less/button-groups.less

@@ -136,6 +136,7 @@
 .success {
   .caret {
     border-top-color: @white;
+    border-bottom-color: @white;
     .opacity(75);
   }
 }